Unless I have forgotten something, I do not know a good explanation for where a new, 1 eV scale of physics (e.g. as the mass of a sterile neutrino) would come from. Sheppeard has an idea but it relies on a particular mass spectrum for the active neutrinos, that I don't find compelling.
Reality of a 1 eV neutrino seems to require baroque interactions. They would also be needed to satisfy cosmological constraints (section 3).
